The present invention relates to force and acceleration measurement devices and methods, and in particular to micro-machined electromechanical sensor (MEMS) force and acceleration measurement devices employing circular ring diaphragm flexures in a small, rugged device having high pick-off sensitivity.
The manufacture of micro-machined electromechanical sensor (MEMS) force and acceleration measurement devices is generally well-known for many different applications. Some applications require the device to provide very accurate measurements, while other applications require the device to withstand extreme shock and vibration environments.
Some applications require the measurement of force in extreme dynamic environments. For example, if a gun-launched projectile requires on-board acceleration sensing, the accelerometer providing the sensing must have a high pickoff sensitivity, and must be capable of operating in a high-G range with high-G shock survivability characteristics. An accelerometer in a gun-launch application must also exhibit low cross-axis sensitivity characteristics. All of these features must be contained in a low cost, small size accelerometer device. Unfortunately, known accelerometer devices lack one or more of the above features.
The present invention provides an inexpensive force measurement device having high pick-off sensitivity in a high-G input range which can operate in a high-G shock environment by providing, in contrast to the prior art devices and methods, a capacitance pick-off force sensor having a proof mass with spaced-apart tooth-type electrodes that is suspended by an annular suspension member. The device of the present invention provides easily implemented fabrication modification for trading-off between input range and pick-off sensitivity by altering etching periods of the annular suspension member. Alternatively, the input range and pick-off sensitivity can be traded-off by enlarging or reducing the area of the annular suspension member.
The apparatus and method of the present invention provide a force-sensing device having a cover plate and a proof mass, wherein the cover plate includes an inner portion and an outer portion, the inner portion is formed with a plurality of first spaced apart electrodes projecting therefrom that define first spaces therebetween; and the proof mass includes an inner portion that is formed with a plurality of second spaced apart electrodes projecting therefrom that define second spaces therebetween, an outer portion that is coupled to the outer portion of the cover plate with the second electrodes being electrically isolated from the first electrodes, and the second electrodes and spaces are aligned with the first electrodes and spaces such that, when the inner portion of the proof mass is deflected toward the inner portion of the cover plate, the second electrodes pass into the first spaces and the first electrodes pass into the second spaces, and a flexible suspension member that is coupled between its inner and outer portions.
According to one aspect of the invention, the force-sensing device of the invention is embodied having an annular flexure and electrodes in the cover plate and proof mass that are structured as cooperating pluralities of overlapping concentric rings.
According to another aspect of the invention, the force-sensing device of the invention is embodied as a double-layer force sensor formed of first and second substantially round semiconductor substrates each having substantially planar and parallel opposing offset top and bottom surfaces; a bottom cover plate is formed in the first substrate, the bottom cover plate including: a pattern of upright and spaced apart electrodes projecting from a central portion of the top surface, and an upright annular ridge portion projecting from a peripheral edge portion of the top surface; and a proof mass is formed in the second substrate, the proof mass including: a cooperating upright annular ridge portion projecting from a peripheral edge portion of the bottom surface and being fixed to the ridge portion of the bottom cover plate, a central portion flexibly suspended from the annular ridge portion, and a cooperating pattern of upright and spaced apart electrodes projecting from the central portion of the bottom surface and offset relative to the pattern of electrodes on the top surface of the bottom cover plate such that the cooperating pattern of electrodes passes between the pattern of electrodes on the bottom cover plate when the cooperating annular ridge portion of the proof mass is engaged with the annular ridge portion on the top surface of the bottom cover plate.
According to another aspect of the invention, the proof mass is formed as an annular flexure suspending the central portion from the annular ridge portion.
According to another aspect of the invention, the pattern of electrodes on the top surface of the bottom cover plate and the cooperating pattern of electrodes on the bottom surface of the proof mass are each further formed as a concentric pattern of circular electrodes.
According to another aspect of the invention, the cooperating annular ridge portion on the bottom surface of the proof mass is fixed with the annular ridge portion of the top surface of the bottom cover plate by an insulating bonding agent.
According to yet another aspect of the invention, the force-sensing device of the invention is embodied as a three-layer force sensor, having a proof mass positioned between first and second cover plates for closed loop operation. Accordingly, the three-layer force sensor is formed of first and second cover plates each formed in respective first and second substantially round semiconductor substrates having substantially planar and parallel opposing offset first and second surfaces, one of the first and second surfaces of each of the first and second cover plates having an annular bonding region, and a central portion positioned within the annular bonding portion and having a plurality of upright and spaced apart electrodes projecting therefrom; and a proof mass positioned between the first surface of the first cover plate and the first surface of the second cover plate, the proof mass being formed in a third substantially round semiconductor substrate having substantially planar and parallel opposing offset first and second surfaces, each of the first and second surfaces having an annular bonding region, each of the annular bonding regions on the first and second proof mass surfaces being bonded to the annular bonding region of one of the first and second cover plates, a central portion positioned within the annular bonding portion and having a plurality of upright and spaced apart electrodes projecting therefrom, each of the electrodes projecting from the first side of the proof mass being aligned with interstices formed between the spaced part electrodes projecting from the surface of the first cover plate, and each of the electrodes projecting from the second side of the proof mass being aligned with interstices formed between the spaced apart electrodes projecting from the surface of the second cover plate, and an integral annular suspension member suspending the central portion from the annular bonding region.
According to still other aspects of the invention, a method is provided for measuring a force input along a measurement axis, the method includes electrically isolating a first pattern of upright electrodes relative to a second pattern of upright electrodes; suspending the first pattern of electrodes relative to the second pattern of electrodes for motion of the first electrodes into recesses between the second electrodes; generating a capacitance between the first and second electrodes; changing capacitance as a function of a displacement of the first pattern of electrodes relative to the second pattern of electrodes; and measuring the capacitance change.
According to another aspect of the method of the invention, suspending the first pattern of electrodes for motion relative to the second pattern of electrodes includes suspending the first pattern of electrodes for motion substantially along a measurement axis.
According to another aspect of the method of the invention, suspending the first pattern of electrodes for motion relative to the second pattern of electrodes includes substantially limiting motion of the first pattern of electrodes to motion along a measurement axis.
According to another aspect of the method of the invention, the method also includes limiting the motion of the first pattern of electrodes relative to the second pattern of electrodes.
According to still another aspect of the method of the invention, the method also includes electrically isolating a third pattern of upright electrodes relative to a fourth pattern of upright electrodes; suspending the third pattern of electrodes in combination with the first pattern of electrodes and relative to the fourth pattern of electrodes for motion of the third electrodes into recesses between the fourth electrodes; generating a capacitance between the third and fourth electrodes; changing capacitance as a function of a displacement of the third pattern of electrodes relative to the fourth pattern of electrodes; and measuring the capacitance change due to displacement of the third pattern of electrodes relative to the fourth pattern of electrodes.